Planetary gear train of automatic transmission for a vehicle

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving rotary power of an engine, an output gear outputting the rotary power with rotary speed changed, a first planetary gear set, a second planetary gear set, a third planetary gear set, a fourth planetary gear set, a first rotating shaft, a second rotating shaft, a third rotating shaft, a fourth rotating shaft selectively connected with the third rotating shaft and a transmission housing simultaneously, a fifth rotating shaft selectively connected with the first rotating shaft and the input shaft simultaneously, a sixth rotating shaft, a seventh rotating shaft selectively connected with the sixth rotating shaft and the transmission housing simultaneously, an eighth rotating shaft selectively connected with the sixth rotating shaft and the output gear simultaneously, and six friction elements.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent ApplicationNumber 10-2015-0077998 filed Jun. 2, 2015, the entire contents of whichis incorporated herein for all purposes by this reference.

BACKGROUND OF INVENTION

Field of the Invention

The present invention relates to an automatic transmission for avehicle, and more particularly, to a planetary gear train of anautomatic transmission for a vehicle which implements forward 11-speedsby a minimum configuration to improve power transmission performance andfuel efficiency.

Description of Related Art

Recently, increasing oil prices have caused vehicle manufacturers allover the world to rush into infinite competition. Particularly in thecase of engines, manufacturers have been pursuing efforts to reduce theweight and improve fuel efficiency of vehicles by reducing engine size,etc.

As a result, research into reduction of weight and enhancement of fuelefficiency through downsizing has been conducted in the case of anengine and research for simultaneously securing operability and fuelefficiency competitiveness through multiple speed stages has beenconducted in the case of an automatic transmission.

However, in the automatic transmission, as the number of transmissionspeed stages increases, the number of internal components alsoincreases, and as a result, the automatic transmission may be difficultto mount, the manufacturing cost and weight may be increased, and powertransmission efficiency may be deteriorated.

Accordingly, development of a planetary gear train which may bring aboutmaximum efficiency with a small number of components may be important inorder to increase a fuel efficiency enhancement effect through themultiple speed stages.

In this aspect, in recent years, 8 and 9-speed automated transmissionstend to be implemented and the research and development of a planetarygear train capable of implementing more transmission steps has also beenactively conducted.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission for a vehicle thatimplements forward 11-speeds and reverse 1-speed transmission stageswith a minimum configuration and improves power transmission efficiencyand fuel efficiency through multiple speed stages and quietness ofdriving by using operation point in a range of low rotation speed of anengine.

In various aspects of the present invention, a planetary gear train ofan automatic transmission for a vehicle may include an input shaftreceiving rotary power of an engine, an output gear outputting therotary power with rotary speed changed, a first planetary gear setincluding a first sun gear, a first planetary carrier, and a first ringgear, a second planetary gear set including a second sun gear, a secondplanetary carrier, and a second ring gear, a third planetary gear setincluding a third sun gear, a third planetary carrier, and a third ringgear, a fourth planetary gear set including a fourth sun gear, a fourthplanetary carrier, and a fourth ring gear, a first rotating shaftincluding the first sun gear, a second rotating shaft including thefirst planetary carrier, the second ring gear, the third planetarycarrier, and the fourth ring gear, a third rotating shaft including thefirst ring gear and the third ring gear, a fourth rotating shaftincluding the second sun gear and selectively connected with the thirdrotating shaft and a transmission housing simultaneously, a fifthrotating shaft including the second planetary carrier and selectivelyconnected with the first rotating shaft and the input shaftsimultaneously, a sixth rotating shaft including the third sun gear, aseventh rotating shaft including the fourth sun gear and selectivelyconnected with the sixth rotating shaft and the transmission housingsimultaneously, an eighth rotating shaft including the fourth planetarycarrier and selectively connected with the sixth rotating shaft and theoutput gear simultaneously, and six friction elements disposed atportions selectively connecting the rotating shafts or selectivelyconnecting the rotating shafts and the transmission housingrespectively.

Each of the first, the second, the third, and the fourth planetary gearset comprises a single-pinion planetary gear set.

The first, the second, the third, and the fourth planetary gear set maybe sequentially disposed from an engine side.

The six friction elements include a first clutch interposed between thefirst rotating shaft and the fifth rotating shaft, a second clutchinterposed between the sixth rotating shaft and the eighth rotatingshaft, a third clutch interposed between the third rotating shaft andthe fourth rotating shaft, a fourth clutch interposed between the sixthrotating shaft and the seventh rotating shaft, a first brake interposedbetween the fourth rotating shaft and the transmission housing, and asecond brake interposed between the seventh rotating shaft and thetransmission housing.

Transmission stages implemented by selectively operating the sixfriction elements may include a first forward transmission stageimplemented by simultaneously operating the third and the fourthclutches and the second brake, a second forward transmission stageimplemented by simultaneously operating the second and the thirdclutches and the second brake, a third forward transmission stageimplemented by simultaneously operating the first and the third clutchesand the second brake, a fourth forward transmission stage implemented bysimultaneously operating the first, the second, and the third clutches,a fifth forward transmission stage implemented by simultaneouslyoperating the first and the second clutches and the second brake, asixth forward transmission stage implemented by simultaneously operatingthe first and the second clutches and the first brake, a seventh forwardtransmission stage implemented by simultaneously operating the firstclutch and the first and the second brakes, an eighth forwardtransmission stage implemented by simultaneously operating the first andthe fourth clutches and the first brake, a ninth forward transmissionstage implemented by simultaneously operating the second and the fourthclutches and the first brake, a tenth forward transmission stageimplemented by simultaneously operating the third and the fourthclutches and the first brake, an eleventh forward transmission stageimplemented by simultaneously operating the second and the thirdclutches and the first brake, and a reverse transmission stageimplemented by simultaneously operating the first and the fourthclutches and the second brake.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft receiving rotary power of an engine, an output gear outputting therotary power with rotary speed changed, a first planetary gear setincluding a first sun gear, a first planetary carrier, and a first ringgear, a second planetary gear set including a second sun gear, a secondplanetary carrier, and a second ring gear, a third planetary gear setincluding a third sun gear, a third planetary carrier, and a third ringgear, a fourth planetary gear set including a fourth sun gear, a fourthplanetary carrier, and a fourth ring gear, a first clutch selectivelyconnecting the first sun gear and the second planetary carrier, a secondclutch selectively connecting the third sun gear and the fourthplanetary carrier, a third clutch selectively connecting the first ringgear and the second sun gear, a fourth clutch selectively connecting thethird sun gear and the fourth sun gear, a first brake selectivelyconnecting the second sun gear with a transmission housing, a secondbrake selectively connecting the fourth sun gear with the transmissionhousing, in which the input shaft may be directly connected with thesecond planetary carrier, the output gear may be directly connected withthe fourth planetary carrier, the first planetary carrier, the secondring gear, the third planetary carrier, and the fourth ring gear aredirectly connected, and the first ring gear and the third ring gear maybe directly connected.

It is understood that the term “vehicle” or “vehicular” or other similarterms as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuel derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example, bothgasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

FIG. 2 is an operation table for each of transmission steps ofrespective friction elements applied to an exemplary planetary geartrain according to the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

FIG. 1 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

Referring to FIG. 1, the planetary gear train according to variousembodiments of the present invention includes a first, a second, athird, and a fourth planetary gear set PG1, PG2, PG3, and PG4 disposedon a same axis line, an input shaft IS, an output shaft OS, eightrotating shafts TM1 to TM8 that connect respective rotating elements ofthe first, second, third, and fourth planetary gear sets PG1, PG2, PG3,and PG4 to each other, six friction elements C1 to C4 and B1 and B2, anda transmission housing H.

As a result, rotary power input from the input shaft IS is transmittedby an inter-complementation operation of the first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4 to be output throughthe output gear OG with rotary speed changed.

In addition, the respective simple planetary gear sets are disposed inan order of the first, the second, the third, and the fourth planetarygear set PG1, PG2, PG3, and PG4 starting from an engine side.

The input shaft IS is an input member and rotary power from a crankshaftof an engine is torque-converted through a torque converter to be inputinto the input shaft IS.

The output gear OG as an output member transmits transmitted drivingpower to a driving shaft through a differential.

The first planetary gear set PG1, which is a single-pinion planetarygear set, includes a first sun gear S1, a first planetary carrier PC1that rotatably supports a first pinion P1 which engages externally withthe first sun gear S1, and a first ring gear R1 which engages internallywith the first pinion P1 as rotating elements.

The second planetary gear set PG2, which is a single-pinion planetarygear set, includes a second sun gear S2, a second planetary carrier PC2that rotatably supports a second pinion P2 which engages externally withthe second sun gear S2, and a second ring gear R2 which engagesinternally with the second pinion P2.

The third planetary gear set PG3, which is a single-pinion planetarygear set, includes a third sun gear S3, a third planetary carrier PC3that rotatably supports a third pinion P3 which engages externally withthe third sun gear S3, and a third ring gear R3 which engages internallywith the third pinion P3.

The fourth planetary gear set PG4, which is a single-pinion planetarygear set, includes a fourth sun gear S4, a fourth planetary carrier PC4that rotatably supports a fourth pinion P4 which engages externally withthe fourth sun gear S4, and a fourth ring gear R4 which engagesinternally with the fourth pinion P4.

In the first, second, third, and fourth planetary gear sets PG1, PG2,PG3, and PG4, one or more rotating elements are directly connected toeach other to operate with a total of eight rotating shafts TM1 to TM8.

Configurations of the eight rotating shafts TM1 to TM8 will be describedbelow.

The first rotating shaft TM1 is configured to include the first sun gearS1.

The second rotating shaft TM2 is configured to include the firstplanetary carrier PC1, the second ring gear R2, the third planetarycarrier PC3, and the fourth ring gear R4.

The third rotating shaft TM3 is configured to include the first ringgear R1 and the third ring gear R3.

The fourth rotating shaft TM4 is configured to include the second sungear S2 and is selectively connected with the third rotating shaft TM3and a transmission housing H simultaneously.

The fifth rotating shaft TM5 is configured to include the secondplanetary carrier PC2 and operates all the time as an input element bybeing selectively connected with the first rotating shaft TM1 anddirectly connected with the input shaft IS simultaneously.

The sixth rotating shaft TM6 is configured to include the third sun gearS3.

The seventh rotating shaft TM7 is configured to include the fourth sungear S4 and is selectively connected with the sixth rotating shaft TM6and the transmission housing H simultaneously.

The eighth rotating shaft TM8 is configured to include the fourthplanetary carrier PC4 and operates all the time as an output element bybeing selectively connected with the sixth rotating shaft TM6 anddirectly connected with the output gear OG simultaneously.

In addition, four clutches C1, C2, C3, and C4 which are frictionelements are disposed at portions selectively connecting the rotatingshafts respectively, among the rotating shafts TM1 to TM8.

Further, two brakes B1 and B2 which are other friction elements aredisposed at portions selectively connecting the transmission housing Hand the rotating shafts respectively, among the rotating shafts TM1 toTM8.

Layout positions of the six friction elements C1 to C4 and B1 to B2 willbe described below.

The first clutch C1 is interposed between the first rotating shaft TM1and the fifth rotating shaft TM5 and operates such that the firstrotating shaft TM1 and the fifth rotating shaft TM5 selectively operateas one body.

The second clutch C2 is interposed between the sixth rotating shaft TM6and the eighth rotating shaft TM8 and operates such that the sixthrotating shaft TM6 and the eighth rotating shaft TM8 selectively operateas one body.

The third clutch C3 is interposed between the third rotating shaft TM3and the fourth rotating shaft TM4 and operates such that the thirdrotating shaft TM3 and the fourth rotating shaft TM4 selectively operateas one body.

The fourth clutch C4 is interposed between the sixth rotating shaft TM6and the seventh rotating shaft TM7 and operates such that the sixthrotating shaft TM6 and the seventh rotating shaft TM7 selectivelyoperate as one body.

The first brake B1 is interposed between the fourth rotating shaft TM4and the transmission housing H and operates such that the fourthrotating shaft TM4 can selectively operate as a fixing element.

The second brake B2 is interposed between the seventh rotating shaft TM7and the transmission housing H and operates such that the seventhrotating shaft TM7 can selectively operate as a fixing element.

The respective friction elements constituted by the first, the second,the third, and the fourth clutch C1, C2, C3, and C4 and the first andthe second brake B1 and B2 as described above may be configured bymultiple-disk hydraulic friction joining units which are friction-joinedby hydraulic pressure.

FIG. 2 is an operation table for each of transmission steps ofrespective friction elements applied to an exemplary planetary geartrain according to the present invention.

As illustrated in FIG. 2, in an exemplary planetary gear train accordingto the present invention, while three friction elements operate in therespective transmission steps, a transmission is performed.

A first forward transmission step 1ST is implemented by simultaneouslyoperating the third and the fourth clutch C3 and C4 and the second brakeB2.

A second forward transmission step 2ND is implemented by simultaneouslyoperating the second and the third clutch C2 and C3 and the second brakeB2.

A third forward transmission step 3RD is implemented by simultaneouslyoperating the first and the third clutch C1 and C3 and the second brakeB2.

A fourth forward transmission step 4TH is implemented by simultaneouslyoperating the first, the second, and the third clutch C1, C2, and C3.

A fifth forward transmission step 5TH is implemented by simultaneouslyoperating the first and the second clutch C1 and C2 and the second brakeB2.

A sixth forward transmission step 6TH is implemented by simultaneouslyoperating the first and the second clutch C1 and C2 and the first brakeB1.

A seventh forward transmission step 7TH is implemented by simultaneouslyoperating the first clutch C1 and the first and the second brake B1 andB2.

An eighth forward transmission step 8TH is implemented by simultaneouslyoperating the first and the fourth clutch C1 and C4 and the first brakeB1.

A ninth forward transmission step 9TH is implemented by simultaneouslyoperating the second and the fourth clutch C2 and C4 and the first brakeB1.

A tenth forward transmission step 10TH is implemented by simultaneouslyoperating the third and the fourth clutch C3 and C4 and the first brakeB1.

An eleventh forward transmission step 11TH is implemented bysimultaneously operating the second and the third clutch C2 and C3 andthe first brake B1.

A reverse transmission step REV is implemented by simultaneouslyoperating the first and the fourth clutch C1 and C4 and the second brakeB2.

The above transmission processes will be described more in detail below.

In the first forward transmission step 1ST, when the third and thefourth clutch C3 and C4 and the second brake B2 simultaneously operate,an input is made into the fifth rotating shaft TM5 in a state of thethird rotating shaft TM3 being connected with the fourth rotating shaftTM4 and the sixth rotating shaft TM6 being connected with the seventhrotating shaft TM7 and the first forward transmission step 1ST isperformed while the seventh rotating shaft TM7 operates as a fixingelement.

In the second forward transmission step 2ND, when the second and thethird clutch C2 and C3 and the second brake B2 simultaneously operate,an input is made into the fifth rotating shaft TM5 in a state of thesixth rotating shaft TM6 being connected with the eighth rotating shaftTM8 and the third rotating shaft TM3 being connected with the fourthrotating shaft TM4 and the second forward transmission step 2ND isperformed while the seventh rotating shaft TM7 operates as a fixingelement.

In the third forward transmission step 3RD, when the first and the thirdclutch C1 and C3 and the second brake B2 simultaneously operate, aninput is made into the fifth rotating shaft TM5 in a state of the firstrotating shaft TM 1 being connected with the fifth rotating shaft TM5and the third rotating shaft TM3 being connected with the fourthrotating shaft TM4 and the third forward transmission step 3RD isperformed while the seventh rotating shaft TM7 operates as a fixingelement.

In the fourth forward transmission step 4TH, when the first, the second,the third clutch C1, C2, and C3 simultaneously operate, an input is madeinto the fifth rotating shaft TM5 in a state of the first rotating shaftTM1 being connected with the fifth rotating shaft TM5, the sixthrotating shaft TM6 being connected with the eighth rotating shaft TM8,and the third rotating shaft TM3 being connected with the fourthrotating shaft TM4 and the fourth forward transmission step 4TH in whichthe input is intactly output, is performed while all the planetary gearsets become serially connected.

In the fifth forward transmission step 5TH, when the first and thesecond clutch C1 and C2 and the second brake B2 simultaneously operate,an input is made into the fifth rotating shaft TM5 in a state of thefirst rotating shaft TM1 being connected with the fifth rotating shaftTM5 and the sixth rotating shaft TM6 being connected with the eighthrotating shaft TM8 and the fifth forward transmission step 5TH isperformed while the seventh rotating shaft TM7 operates as a fixingelement.

In the sixth forward transmission step 6TH, when the first and thesecond clutch C1 and C2 and the first brake B1 simultaneously operate,an input is made into the fifth rotating shaft TM5 in a state of thefirst rotating shaft TM1 being connected with the fifth rotating shaftTM5 and the sixth rotating shaft TM6 being connected with the eighthrotating shaft TM8 and the sixth forward transmission step 6TH isperformed while the fourth rotating shaft TM4 operates as a fixingelement.

In the seventh forward transmission step 7TH, when the first clutch C1and the first and the second brake B1 and B2 simultaneously operate, aninput is made into the fifth rotating shaft TM5 in a state of the firstrotating shaft TM1 being connected with the fifth rotating shaft TM5 andthe seventh forward transmission step 7TH is performed while the fourthand the seventh rotating shaft TM4 and TM7 operate as fixing elements.

In the eighth forward transmission step 8TH, when the first and thefourth clutch C1 and C4 and the first brake B1 simultaneously operate,an input is made into the fifth rotating shaft TM5 in a state of thefirst rotating shaft TM1 being connected with the fifth rotating shaftTM5 and the sixth rotating shaft TM6 being connected with the seventhrotating shaft TM7 and the eighth forward transmission step 8TH isperformed while the fourth rotating shaft TM4 operates as a fixingelement.

In the ninth forward transmission step 9TH, when the second and thefourth clutch C2 and C4 and the first brake B1 simultaneously operate,an input is made into the fifth rotating shaft TM5 in a state of thesixth rotating shaft TM6 being connected with the seventh and the eighthrotating shaft TM7 and TM8 and the ninth forward transmission step 9THis performed while the fourth rotating shaft TM4 operates as a fixingelement.

In the tenth forward transmission step 10TH, when the third and thefourth clutch C3 and C4 and the first brake B1 simultaneously operate,an input is made into the fifth rotating shaft TM5 in a state of thethird rotating shaft TM3 being connected with the fourth rotating shaftTM4 and the sixth rotating shaft TM6 being connected with the seventhrotating shaft TM7 and the tenth forward transmission step 10TH isperformed while the fourth rotating shaft TM4 operates as a fixingelement.

In the eleventh forward transmission step 11TH, when the second and thethird clutch C2 and C3 and the first brake B1 simultaneously operate, aninput is made into the fifth rotating shaft TM5 in a state of the sixthrotating shaft TM6 being connected with the eighth rotating shaft TM8and the third rotating shaft TM3 being connected with the fourthrotating shaft TM4 and the eleventh forward transmission step 11TH isperformed while the fourth rotating shaft TM4 operates as a fixingelement.

In the reverse transmission step REV, when the first and the fourthclutch C1 and C4 and the second brake B2 simultaneously operate, aninput is made into the fifth rotating shaft TM5 in a state of the firstrotating shaft TM1 being connected with the fifth rotating shaft TM5 andthe sixth rotating shaft TM6 being connected with the seventh rotatingshaft TM7 and the reverse transmission step REV is performed while theseventh rotating shaft TM7 operates as a fixing element.

As described above, in the planetary gear train according to variousembodiments of the present invention, four planetary gear sets PG1, PG2,PG3, and PG4 may implement the forward 11-speed and reverse 1-speedtransmission steps through the operation-control of four clutches C1,C2, C3, and C4 and two brakes B1 and B2.

As a result, the planetary gear train according to various embodimentsof the present invention may improve the power transmission efficiencyand the fuel efficiency through the multiple speed stages of theautomatic transmission.

Further, quietness of driving can be also much improved becausetransmission steps suitable for rotation speed of an engine can beimplemented through the multiple speed stages of the automatictransmission.

In addition, three friction elements operate for each transmission step,and as a result, a friction drag loss is decreased by minimizing thenumber of friction elements which do not operate, thereby improving thepower transmission efficiency and the fuel efficiency.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

1. A planetary gear train of an automatic transmission for a vehiclecomprising: an input shaft receiving rotary power of an engine; anoutput gear outputting the rotary power with rotary speed changed; afirst planetary gear set including a first sun gear, a first planetarycarrier, and a first ring gear; a second planetary gear set including asecond sun gear, a second planetary carrier, and a second ring gear; athird planetary gear set including a third sun gear, a third planetarycarrier, and a third ring gear; a fourth planetary gear set including afourth sun gear, a fourth planetary carrier, and a fourth ring gear; afirst rotating shaft including the first sun gear; a second rotatingshaft including the first planetary carrier, the second ring gear, thethird planetary carrier, and the fourth ring gear; a third rotatingshaft including the first ring gear and the third ring gear; a fourthrotating shaft including the second sun gear and selectively connectedwith the third rotating shaft and a transmission housing simultaneously;a fifth rotating shaft including the second planetary carrier andselectively connected with the first rotating shaft and the input shaftsimultaneously; a sixth rotating shaft including the third sun gear; aseventh rotating shaft including the fourth sun gear and selectivelyconnected with the sixth rotating shaft and the transmission housingsimultaneously; an eighth rotating shaft including the fourth planetarycarrier and selectively connected with the sixth rotating shaft and theoutput gear simultaneously; and six friction elements disposed atportions selectively connecting the rotating shafts or selectivelyconnecting the rotating shafts and the transmission housingrespectively.
 2. The planetary gear train of claim 1, wherein each ofthe first, the second, the third, and the fourth planetary gear setcomprises a single-pinion planetary gear set.
 3. The planetary geartrain of claim 1, wherein the first, the second, the third, and thefourth planetary gear set are sequentially disposed from an engine side.4. The planetary gear train of claim 1, wherein the six frictionelements comprise: a first clutch interposed between the first rotatingshaft and the fifth rotating shaft; a second clutch interposed betweenthe sixth rotating shaft and the eighth rotating shaft; a third clutchinterposed between the third rotating shaft and the fourth rotatingshaft; a fourth clutch interposed between the sixth rotating shaft andthe seventh rotating shaft; a first brake interposed between the fourthrotating shaft and the transmission housing; and a second brakeinterposed between the seventh rotating shaft and the transmissionhousing.
 5. The planetary gear train of claim 4, wherein transmissionstages implemented by selectively operating the six friction elementsinclude: a first forward transmission stage implemented bysimultaneously operating the third and the fourth clutches and thesecond brake; a second forward transmission stage implemented bysimultaneously operating the second and the third clutches and thesecond brake; a third forward transmission stage implemented bysimultaneously operating the first and the third clutches and the secondbrake; a fourth forward transmission stage implemented by simultaneouslyoperating the first, the second, and the third clutches; a fifth forwardtransmission stage implemented by simultaneously operating the first andthe second clutches and the second brake; a sixth forward transmissionstage implemented by simultaneously operating the first and the secondclutches and the first brake; a seventh forward transmission stageimplemented by simultaneously operating the first clutch and the firstand the second brakes; an eighth forward transmission stage implementedby simultaneously operating the first and the fourth clutches and thefirst brake; a ninth forward transmission stage implemented bysimultaneously operating the second and the fourth clutches and thefirst brake; a tenth forward transmission stage implemented bysimultaneously operating the third and the fourth clutches and the firstbrake; an eleventh forward transmission stage implemented bysimultaneously operating the second and the third clutches and the firstbrake; and a reverse transmission stage implemented by simultaneouslyoperating the first and the fourth clutches and the second brake.
 6. Aplanetary gear train of an automatic transmission for a vehiclecomprising: an input shaft receiving rotary power of an engine; anoutput gear outputting the rotary power with rotary speed changed; afirst planetary gear set including a first sun gear, a first planetarycarrier, and a first ring gear; a second planetary gear set including asecond sun gear, a second planetary carrier, and a second ring gear; athird planetary gear set including a third sun gear, a third planetarycarrier, and a third ring gear; a fourth planetary gear set including afourth sun gear, a fourth planetary carrier, and a fourth ring gear; afirst clutch selectively connecting the first sun gear and the secondplanetary carrier; a second clutch selectively connecting the third sungear and the fourth planetary carrier; a third clutch selectivelyconnecting the first ring gear and the second sun gear; a fourth clutchselectively connecting the third sun gear and the fourth sun gear; afirst brake selectively connecting the second sun gear with atransmission housing; a second brake selectively connecting the fourthsun gear with the transmission housing, wherein the input shaft isdirectly connected with the second planetary carrier, the output gear isdirectly connected with the fourth planetary carrier, the firstplanetary carrier, the second ring gear, the third planetary carrier,and the fourth ring gear are directly connected, and the first ring gearand the third ring gear are directly connected.
 7. The planetary geartrain of claim 6, wherein the first, the second, the third, and thefourth planetary gear set are sequentially disposed from an engine side.8. The planetary gear train of claim 6, wherein each of the first, thesecond, the third, and the fourth planetary gear set comprises asingle-pinion planetary gear set.
 9. The planetary gear train of claim8, wherein the first, the second, the third, and the fourth planetarygear set are sequentially disposed starting from an engine side.
 10. Theplanetary gear train of claim 6, wherein transmission stages implementedby selectively operating the first, the second, the third, and thefourth clutches and the first and the second brakes include: a firstforward transmission stage implemented by simultaneously operating thethird and the fourth clutches and the second brake; a second forwardtransmission stage implemented by simultaneously operating the secondand the third clutches and the second brake; a third forwardtransmission stage implemented by simultaneously operating the first andthe third clutches and the second brake; a fourth forward transmissionstage implemented by simultaneously operating the first, the second, andthe third clutches; a fifth forward transmission stage implemented bysimultaneously operating the first and the second clutches and thesecond brake; a sixth forward transmission stage implemented bysimultaneously operating the first and the second clutches and the firstbrake; a seventh forward transmission stage implemented bysimultaneously operating the first clutch and the first and the secondbrakes; an eighth forward transmission stage implemented bysimultaneously operating the first and the fourth clutches and the firstbrake; a ninth forward transmission stage implemented by simultaneouslyoperating the second and the fourth clutches and the first brake; atenth forward transmission stage implemented by simultaneously operatingthe third and the fourth clutches and the first brake; an eleventhforward transmission stage implemented by simultaneously operating thesecond and the third clutches and the first brake; and a reversetransmission stage implemented by simultaneously operating the first andthe fourth clutches and the second brake.
 11. The planetary gear trainof claim 2, wherein the first, the second, the third, and the fourthplanetary gear set are sequentially disposed from an engine side.